1. Field of the Invention
The present invention relates to a passivated and/or painted steel sheet using a coated steel sheet having a hot-dip Alxe2x80x94Zn base coating layer containing 20 to 95 mass % Al, and a method for manufacturing the same.
2. Description of the Related Arts
Steel sheets coated by a hot-dip Alxe2x80x94Zn base coating layer containing 20 to 90 mass % Al give superior corrosion resistance, as described in JP-B-46-7161, (the term xe2x80x9cJP-Bxe2x80x9d referred herein signifies the xe2x80x9cExamined Japanese patent publicationxe2x80x9d), to hot-dip galvanized steel sheets. Owing to the advantageous property, the coated steel sheets increase in demand in recent years centering on the building materials.
For manufacturing that type of coated steel sheet, a hot-rolled steel sheet is pickled and descaled, or further is cold-rolled to prepare a substrate steel sheet. Thus prepared substrate steel sheet is introduced to a continuous hot-dip coating apparatus, where the following-given treatment is applied thereto.
First, the substrate steel sheet enters an annealing furnace which is kept to a reducing atmosphere, where the steel sheet is heated to a specified temperature to undergo annealing treatment. During the course of annealing, rolling oil or the like attached to the surface of the substrate steel sheet is removed, and oxide film formed thereon is reduced and removed. After that, the substrate steel sheet passes through a snout immersed at the bottom thereof in a coating bath, and enters a hot-dip galvanizing bath containing a specified amount of aluminum. The substrate steel sheet which is immersed in the coating bath is then pulled up therefrom via a sink roll, and the coating weight on the substrate steel sheet is adjusted by injecting a pressurized gas from gas-wiping nozzles, arranged above the coating bath, against the surface of the coated steel sheet. Then, the coated steel sheet is cooled in a cooling unit to obtain the hot-dip Alxe2x80x94Zn base coated steel sheet having a specified coating layer thereon.
For assuring specified quality and material properties of coating layer, the continuous hot-dip coating apparatus is precisely controlled within a predetermined control range, in terms of heat treatment condition and atmospheric condition of the annealing furnace, and operating conditions such as composition of coating bath liquor and cooling rate after coated.
The coating layer of thus prepared coated steel sheet has a portion of dendrites of Al which contains mainly Zn to a supersaturation level and a balance portion of gaps between dendrites, which dendrites layer in the direction of coating layer thickness. Owing to the characteristic film structure, the hot-dip Alxe2x80x94Zn base coated steel sheet gives excellent corrosion resistance.
The coating bath normally contains Si to about 1.5 mass %. The Si functions to suppress the growth of alloy phase at interface between the coating layer and the substrate steel sheet, thus the hot-dip Alxe2x80x94Zn base coated steel sheet has around 1 to 2 "igr"m of the alloy phase thickness. Since thinner alloy phase gives more increased portion of the characteristic film structure which provides excellent corrosion resistance, the suppression of growth of the alloy phase contributes to the improved corrosion resistance. The alloy phase is harder than the coating layer and functions as the origin of crack generation during working, so the suppression of growth of alloy phase reduces the crack generation and improves the workability. Since the cracked portion has low resistance to corrosion because of the exposure of substrate steel sheet, the reduction in crack generation also improves the corrosion resistance at worked portion.
The coating bath generally contains inevitable impurities, Fe eluted from steel sheet and from equipment in the bath, and Si added to the bath for suppressing the growth of alloy phase. Other elements than those given above may be added to the coating bath. Those elements exist in the alloy phase and in the coating layer in a form of alloy or single elements.
Practical applications of the hot-dip Alxe2x80x94Zn base coated steel sheets in as-coated state are very rare. These steel sheets are normally treated before use further by passivation or painting on the surface thereof to prepare surface-treated steel sheets.
When a hot-dip Alxe2x80x94Zn base coated steel sheet is worked by bending or the like, it may generate cracks on the coating layer at the worked portion depending on the degree of the working. On this type of coated steel sheet, the alloy phase having about 1 "igr"m to about 2 "igr"m thickness, existing at the interface between the coating layer and the substrate steel sheet, becomes the origin of the cracks, and the dendrite gaps in the coating layer provide the crack propagation route. Accordingly, compared with a hot-dip galvanized steel sheet having the same coating layer thickness and being subjected to the same degree of working, the hot-dip Alxe2x80x94Zn base coated steel sheet likely gives relatively large crack opening. As a result, there occurs a problem of visible crack generation to degrade the appearance of the steel sheet, depending on the degree of working. Although the hot-dip Alxe2x80x94Zn base coated steel sheet has superior corrosion resistance to the hot-dip galvanized steel sheet having the same coating layer thickness therewith, as described above, the hot-dip Alxe2x80x94Zn base coated steel sheet has a drawback in significant reduction of corrosion resistance at the crack-generated portion, where the substrate steel sheet exposes, compared with the portion of no crack generation.
Countermeasures to these problems have been proposed. For example, JP-B-61-28748 discloses a method for improving the ductility of a coated steel sheet by applying a specific heat treatment to a hot-dip Alxe2x80x94Zn base coated steel sheet. Solely that kind of heat treatment in related art, however, fails to sufficiently improve the ductility of the coating layer.
As described above, the hot-dip Alxe2x80x94Zn base coated steel sheets are normally used as the passivation-treated steel sheets which are prepared by applying passivation treatment to the surface thereof or as the coated steel sheets which are prepared by applying coating thereon. From the point of preventing the crack generation at a portion of work such as bending, sole improvement in the ductility of the coating layer to some extent, as disclosed in related art, not necessarily improves directly the performance of the products, or the performance of workability and the corrosion resistance of the worked portion on the surface-treated steel sheet after subjected to passivation treatment or coating, to a practically applicable level.
It is an object of the present invention to provide a coated steel sheet having excellent workability and corrosion resistance at worked portion, and to provide a method for manufacturing same.
To attain the object, the present invention provides a method for manufacturing a coated steel sheet, comprising the steps of: forming an Alxe2x80x94Zn base coating layer containing 20 to 95 mass % Al on a steel sheet by immersing thereof in a hot-dip coating bath; and forming a passivated layer on the coating layer. The method for manufacturing the coated steel sheet includes the step of applying a thermal history to the coating layer.
The step of applying thermal history has the steps of: applying a first thermal history of less than 11xc2x0 C./sec of average cooling rate during the first 10 seconds after the steel sheet left the hot-dip coating bath; and applying a second thermal history of 0.5xc3x97(Txe2x88x92100)(xc2x0 C./hr) or less of average cooling rate thereto in a temperature range of from T(xc2x0 C.) between 130xc2x0 C. and 300xc2x0 C. to 100xc2x0 C.
The step of applying the second thermal history is preferably in the following:
(1) The average cooling rate of the coating layer in a temperature range of from T(xc2x0 C.) between 130xc2x0 C. and 300xc2x0 C. to 100xc2x0 C., after solidification of a hot-dip coated metal, is 0.5xc3x97(Txe2x88x92100)(xc2x0 C./hr) or less.
(2) After solidification of the hot-dip coated metal, the heating is applied to the coating layer to a temperature of T(xc2x0 C.) between 130xc2x0 C. and 300xc2x0 C., followed by applying average cooling rates of 0.5xc3x97(Txe2x88x92100)(xc2x0 C./hr) or less thereto in the temperature range of from T(xc2x0 C.) to 100xc2x0 C.
The step of applying the second thermal history preferably applies a thermal history of average cooling rates of 0.5xc3x97(Txe2x88x92100)(xc2x0 C./hr) or less to the coating layer in a temperature range of from T(xc2x0 C.) between 130xc2x0 C. and 200xc2x0 C. to 100xc2x0 C.
The step of applying the second thermal history is preferably conducted in at least one stage selected from the group consisting of (1) through (4) given below.
(1) before the passivation;
(2) during drying period of the passivation;
(3) after the passivation;
(4) during cooling period after solidification of a hot-dip coated metal.
The step of forming the passivated layer is preferably conducted by applying chromate treatment to the coating layer to form a passivated layer containing chromium of 0.1 mg/m2 or more and less than 100 mg/m2 as metallic chromium. For forming the passivated layer by the chromate treatment, the following-given two methods are preferred.
(1) A chromating solution containing a water organic resin, chromic aid and/or a chromic acid compound which is prepared by reducing a part of chromic aid is applied onto the coating layer. Then, the applied solution is dried at a sheet temperature of from 80xc2x0 C. to 300xc2x0 C. to form the passivated layer. A mass ratio of the organic resin (A) to the chromium (B) as metallic chromium in the passivated layer, (A)/(B), is 1 or more and less than 200. The organic resin in the passivated layer is preferably a thermosetting organic resin.
(2) Chromate treatment is applied to the coating layer. Then a treatment solution containing a water organic resin is applied onto the chromated surface, which is then dried at sheet temperatures of from 80xc2x0 C. to 300xc2x0 C. to form the passivated layer containing organic resin. It is preferable that a mass ratio of the organic resin (A) to the chromium (B) as metallic chromium in the passivated layer, (A)/(B), is 1 or more and less than 200. The organic resin in the passivated layer is preferably a thermosetting organic resin.
As for the method for manufacturing the coated steel sheet, it is preferable that the passivated layer is made of at least one layer that does not contain chromium, and that the coating weight thereof is 0.1 g/m2 or more and less than 5 g/m2. The passivated layer contains an organic resin and at least one compound selected from the group consisting of phosphoric acid, phosphate, silica, silane coupling agent, Ca, Ca-base compound, Mn, Mn-base compound, Mg, Mg-base compound, Ni, Ni-base compound, Co, Co-base compound, Fe, and Fe-base compound.
The method for manufacturing the coated steel sheet preferably further comprises the step of applying at least one paint film layer onto the passivated layer. The step of applying at least one paint film layer onto the passivated layer comprises the steps of: applying a primer onto the passivated layer and baking the primer onto the passivated layer to form a primer layer having thicknesses of 2 "igr"m or more; and applying a top coat onto the primer layer and baking the top coat onto the primer layer to form a top coat layer having a thickness of from 5 to 30 "igr"m.
The primer contains main component resins of a modified polyester resin (C) and a curing agent (D), which modified polyester resin (C) is prepared by a reaction between a polyester resin (A) having number average molecular weights of from 14,000 to 26,000 and having glass transition temperatures of from 0xc2x0 C. to 30xc2x0 C., and an epoxy resin (B), and the modified polyester resin (C) has k-values, given in the following equation, of from 0.7 to 2.5,
xe2x80x83k=[Wbxc3x97(Ma+2xc3x97Mb)]/[(Wa+Wb)xc3x972xc3x97Mb]
where, Wa is the solid matter weight ratio of the polyester resin (A) to [the polyester resin (A)+the epoxy resin (B)], Wb is the solid matter weight ratio of the epoxy resin (B) to [the polyester resin (A)+the epoxy resin (B)], Ma is the number average molecular weight of the polyester resin (A), and Mb is the number average molecular weight of the epoxy resin (B).
The top coat contains main component resins of a polyester resin (E) and a curing agent, which polyester resin (E) has number average molecular weights of from 5,000 to 20,000.
The curing agent (D) existing in the primer is preferably at least one compound selected from the group consisting of amino resin and isocyanate compound.
In the case that the method for manufacturing the coated steel sheet comprises the step of applying painting, it is preferable that the step of applying the second thermal history is conducted in at least one stage selected from the group consisting of (1) through (8) described below:
(1) before the passivation;
(2) during drying period of the passivation;
(3) after the passivation and before the painting of the top coat;
(4) during drying period of the primer;
(5) after the painting of the primer and before the painting of the top coat;
(6) during drying period of the top coat;
(7) after the painting of the top coat;
(8) during cooling period after solidification of the hot-dip coated metal.
According to the method for manufacturing the coated steel sheet, the step of forming the coating layer is preferably the one to form an Alxe2x80x94Zn base coating layer containing 20 to 95 mass % Al and 0.01 to 10 mass % of at least one element selected from the group consisting of Mg, V, and Mn.
The present invention further provides a coated steel sheet having an Alxe2x80x94Zn base coating layer containing 20 to 95 mass % Al and a passivated layer formed on the coating layer.
The coating layer is the one subjected to a first thermal history of less than 11xc2x0 C./sec of average cooling rate during the first 10 seconds after the steel sheet left the hot-dip coating bath, and to a second thermal history of 0.5xc3x97(Txe2x88x92100)(xc2x0 C./hr) or less of average cooling rate in a temperature range of from T(xc2x0 C.) between 130xc2x0 C. and 300xc2x0 C. to 100xc2x0 C.
The second thermal history is preferably in the following.
(1) The average cooling rate in a temperature range of from T(xc2x0 C.) between 130xc2x0 C. and 300xc2x0 C. to 100xc2x0 C., after solidification of the hot-dip coated metal, is 0.5xc3x97(Txe2x88x92100)(xc2x0 C./hr) or less.
(2) After solidification of the hot-dip coated metal, the heating is applied to a temperature T(xc2x0 C.) between 130xc2x0 C. and 300xc2x0 C., followed by applying average cooling rate of 0.5xc3x97(Txe2x88x92100)(xc2x0 C./hr) or less in a temperature range of from T(xc2x0 C.) to 100xc2x0 C.
The step of applying the second thermal history preferably applies a thermal history of average cooling rates of 0.5xc3x97(Txe2x88x92100)(xc2x0 C./hr) or less in a temperature range of from T(xc2x0 C.) between 130xc2x0 C. and 200xc2x0 C. to 100xc2x0 C.
The passivated layer is preferably formed by chromate treatment to contain chromium of 0.1 mg/m2 or more and less than 100 mg/m2 as metallic chromium. For forming the passivated layer by the chromate treatment, the following-given two methods are preferred.
(1) A chromating solution containing a water organic resin, chromic aid and/or a chromic acid compound which is prepared by reducing a part of chromic aid is applied onto the coating layer. Then, the coating layer is dried at sheet temperatures of from 80xc2x0 C. to 300xc2x0 C. to form the passivated layer. The mass ratio of the organic resin (A) to the chromium (B) as metallic chromium in the passivated layer, (A)/(B), is 1 or more and less than 200. The organic resin in the passivated layer is preferably a thermosetting organic resin.
(2) Chromate treatment is applied to the coating layer. Then a treatment solution containing a water organic resin is applied onto the chromated surface, which is then dried at a sheet temperature of from 80xc2x0 C. to 300xc2x0 C. to form the passivated layer containing organic resin. It is preferable that the mass ratio of the organic resin (A) to the chromium (B) as metallic chromium in the passivated layer, (A)/(B), is 1 or more and less than 200.
The organic resin in the passivated layer is preferably a thermosetting organic resin.
In the coated steel sheet, it is preferable that the passivated layer is made of at least one layer that does not contain chromium, and that the coating weight thereof is 0.1 g/m2 or more and less than 5 g/m2. The passivated layer contains an organic resin and at least one compound selected from the group consisting of phosphoric acid, phosphate, silica, silane coupling agent, Ca, Ca-base compound, Mn, Mn-base compound, Mg, Mg-base compound, Ni, Ni-base compound, Co, Co-base compound, Fe, and Fe-base compound.
The coated steel sheet preferably has at least one painting film layer on the passivated layer. The painting film layer contains a primer layer having thicknesses of 2 "igr"m or more formed by applying a primer onto the passivated layer, and a top coat layer having thicknesses of from 5 to 30 "igr"m formed by applying a top coat onto the primer layer.
The primer contains main component resins of a modified polyester resin (C) and a curing agent (D), which modified polyester resin (C) is prepared by a reaction between a polyester resin (A) having number average molecular weights of from 14,000 to 26,000 and having glass transition temperatures of from 0xc2x0 C. to 30xc2x0 C., and an epoxy resin (B), and the modified polyester resin (C) has k-values, given in the following equation, of from 0.7 to 2.5,
k=[Wbxc3x97(Ma+2xc3x97Mb)/[(Wa+Wb)xc3x972xc3x97Mb]
where, Wa is the solid matter weight ratio of the polyester resin (A) to [the polyester resin (A)+the epoxy resin (B)], Wb is the solid matter weight ratio of the epoxy resin (B) to the [the polyester resin (A)+the epoxy resin (B)], Ma is the number average molecular weight of the polyester resin (A), and Mb is the number average molecular weight of the epoxy resin (B).
The top coat contains main component resins of a polyester resin (E) and a curing agent, which polyester resin (E) has number average molecular weights of from 5,000 to 20,000.
The curing agent (D) existing in the primer is preferably at least one compound selected from the group consisting of amino resin and isocyanate compound.
In the coated steel sheet, the coating layer is preferably an Alxe2x80x94Zn base coating layer containing 20 to 95 mass % Al and 0.01 to 10 mass % of at least one element selected from the group consisting of Mg, V, and Mn.